should be possible to mirror the image using
scale(-1,1);

No, it doesn't work...

how do i flip video with this code:

I have a much belated response to deevaniej. I was having this same problem yesterday, and figure I might as well post my solution in case someone else stumbles across this thread.

I flipped the video for this color-tracking example by copying the output of the camera into a separate PImage called camMirror (which has its own array of pixels). I copied the pixels from the camera into camMirror in a for loop, but flipped the x axis around. Finally I drew camMirror with image(camMirror,0,0).

1. import processing.video.*;
2. Capture cam;
3. color trackColor;
4. PImage camMirror;
5. void setup(){
6.   size(1024,768,P2D);
7.   cam = new Capture(this,width,height,30);
8.   cam.start();
9.   camMirror = new PImage(cam.width,cam.height);
10.   trackColor = color(255,0,0);
11.   smooth();
12. }
13. void draw(){
14.   if(cam.available() == true){
15.    cam.read();
16.   }
17.   cam.loadPixels();
18.   //Mirroring the image
19.   for(int x = 0; x < cam.width; x++){
20.     for(int y = 0; y < cam.height; y++){
21.       camMirror.pixels[x+y*cam.width] = cam.pixels[(cam.width-(x+1))+y*cam.width];
22.     }
23.   }
24.   camMirror.updatePixels();
25.   image(camMirror,0,0);
26.   //the closest to the actual color we're tracking
27.   float maxCloseness = 500;
28.   //XY coord for closest color
29.   int closestX = 0;
30.   int closestY = 0;
31.   //Begin loop to walk through every pixel
32.   for(int x = 0; x < camMirror.width; x++){
33.     for(int y = 0; y < camMirror.height; y++){
34.       int loc = x + y*camMirror.width;
35.       //get current color
36.       color currentColor = camMirror.pixels[loc];
37.       float r1 = red(currentColor);
38.       float g1 = green(currentColor);
39.       float b1 = blue(currentColor);
40.       float r2 = red(trackColor);
41.       float g2 = green(trackColor);
42.       float b2 = blue(trackColor);
43.       //euclidean distance to compute colors
44.       float d = dist(r1,g1,b1,r2,g2,b2);  
45.       //if the current color is closer than the last max closeness, get its location
46.       if(d < maxCloseness){
47.         maxCloseness = d;
48.         closestX = x;
49.         closestY = y;
50.       }
51.     }
52.   } 
53.   //Make a threshold; if the color distance is less than 10, draw a circle
54.   if(maxCloseness < 20){
55.     //Draw a circle at the tracked pixel
56.     fill(trackColor);
57.     strokeWeight(4.0);
58.     stroke(0);
59.     ellipse(closestX,closestY,16,16);
60.   }
61. }
62. void mousePressed(){
63.   //Save color that user clicks
64.   int loc = mouseX + mouseY*cam.width;
65.   trackColor = camMirror.pixels[loc];
66. }